The present invention relates to a system for driving a medical device adapted to be used for auxiliary circulation, such as an assisting artificial heart or an intra-aortic balloon pump and, more particularly, to a driving system having a variable assistance ratio. Generally speaking a medical device for auxiliary circulation such as an assisting artificial heart or an intra-aortic balloon pump is used in a patient whose heart is functioning so badly that it cannot maintain necessary blood circulation. A driving system for such a medical device is usually triggered by the cardiogram of the patient. When the heart is restored to its intrinsic function, the medical device is removed from the patient's body. If, however, this removal is made suddenly, it could have an adverse influence upon the body. Therefore, the method of removal of the medical device from the body involves gradually reducing the assistance ratio. This method is usually called "weaning". The driving system of the prior art is exemplified by U.S. Pat. No. 4,016,871 and U.S. Pat. No. 4,175,264. The system, as disclosed, employs the R-waves of the cardiogram. When a trigger signal is inputted, a solenoid pulse signal for supplying positive and negative pressures to the medical device in synchronism with a predetermined timing is outputted to effect diastole and systole of the medical device.
In this system, the limit for the pulse signal is arranged with a frequency divider for carrying out the weaning. By selecting 1/2 or 1/4 as the assistance ratio by means of a push button, the pulse signal is gradually dropped to effect the weaning.
In the driving system disclosed above, however, the assistance ratio has its allowable selection limited to one of 1/1, 1/2, and 1/4, and its change is not divided equally. This makes it difficult to finely cope with the particular state of the patient. Since the disclosed device changes the assistance ratio by dividing the frequency of the pulse signal, it cannot set an assistance ratio between 1/1 and 1/2.